There is some value in 3rd party verifying the system design (the architecture, the protocol, etc.) and general engineering practices in the company, but this still hinges on the need to trust this company not to be (or being coerced to be) malicious. TunnelBear hasn't established the latter, so - yes, there's little to no value in former. There is some marketing value in it though.
PS. Zimmerman's original secure VoIP project was rooted in the idea of reproducible builds. It was open source, but with a license that prohibited any use except for verifying binary builds. It was 20 (?) years ago.